Direct-coupled amplifier



' Dec. 2, 1952 R. P. NELSON DIRECT-COUPLED AMPLIFIER Filed June 29, 1951 INVENTOR. .RUEZRT R 175L500 BY W M Patented Dec. 2, 1952 UNITED STATES PATENT OFFICE DIRECT-COUPLED AMPLIFIER I Robert P. Nelson, Southbridge', Mass, assignor to V Philco Corporation, Philadelphia, Pa., a corporation of Pennsylvania Application'June 29, 1951, Serial No. 234,281

, 1 The invention herein described and claimed relates to direct-coupled amplifiers, sometimes referred to as direct-current amplifiers. However, throughout :this specification the term directcoupled amplifier will be used.

.It is well known that direct-coupled amplifiers tend to be troubled by slow drifts or variations in the D.-C. component of the plate current and in the output voltage level as a result of slow drifts or fluctuations in the .D.-C; supply voltages,

, It is the object of the present invention to provide, in a direct-coupled amplifier, means for minimizing orlsubstantially preventing such output voltage drifts due to variations in power supplyvoltage. 7

- Another object of the present invention is to provide a low-level direct-coupled amplifier having means for compensating for fluctuations in the positive and/or negative power supply volt-, ages, thereby to minimize or substantially prevent resulting variations in the output voltage level.

The above objects are accomplished; in accordance with the present invention, by providing compensation means adapted to maintain substantially. constant,;despite variations in power supply voltages, the D.-C. component of plate current in the second tube of an otherwise conventional two-stage direct-coupled amplifier. More particularly, by the action of the compensation means provided by the present invention, the

Claims, (01. 179-171) 2 eter 23, and a resistor 24 serially connected between the plate of tube l4 and 0-4 The adjustable tap of potentiometer 23 is connected conductively to the controljgrid 25 of thesecond amplifier tube IS. The input signal is applied to the grid-cathode circuit of first amplifier tube [4 and the output of the two-stage direct-coupled amplifier mistaken between the plate of second tube I 5 and ground. The circuit described thus far is conventional and need not be described in further detail.

The circuit [2, provided in accordance with the present invention, comprises atriode whose plate is connected directly to the positive voltage supply, 3+, and whose cathode 3| is connected directlyto the cathode 2| of the second amplifier tube [5. Thus, tubes [5 and 30 are connected to ground through the common cathode biasing resistor 20. A resistance-divider circuit 32 comprising a resistor 33, the resistive element of a potentiometer 34 and a resistor 35 is connected between the positive voltage supply, 13+, and the negative voltage supply, C,:and the adjustable tap of potentiometer 34 is connected conductively to the grid 36 of triode 3i). Triode 30 is chosen of a type having a plate resistance which is small relative to that of second amplifier tube [5 so cathode potential of the second tube of an otherwise conventional two-stage direct-coupled amplifier is caused to vary ina direction and to an extent necessary to neutralize substantially any changes occurring in thegrid or plate potentials of the second tube due to fluctuations in either, or both, of the positive and negative supply volta s. J

The invention will be most readily understood by considering the following detailed description of a preferred embodiment wherein reference is made to the accompanying single figure of draw- In the drawing, I have shown a two-stage direct-coupled amplifier ID to which a circuit [2 has been added inaccordance with the present invention. Except to the extent indicated hereinafter, the direct-coupled amplifier I0 is convem tional and comprises a first triode M, a secondtriode IS, an interstage resistance-divider circuit [6, a source of positive supply voltage-13+, a. resistor l1 connecting the positive voltage source to the plate of first amplifier tube [4, a biasing resistor 18. connected between the cathode of first tube (4 and gro nd, a resistor l9 connecting the positive voltage source to the plate of the second amplifier tube l5, a biasing resistor 2|] connecting the cathode 2| of the second tube Hi to ground, and a source of negative supply voltage, C- The interstage resistance-divider circuit l6 comprises aresistor 22, the resistive element of a potentiomthat, for a givenbias and plate voltage, the D.-C. plate current in tube 30 is much larger than that in tube l5. For example, the plate current in tube 30' may be of the order of ten times as large as that in tube 15. Thus, the cathode potential of tube I5 is determined largely by the plate currentiin tube 30.

In accordance with the preferred form of the present invention, the cathode biasing resistor l8 of first amplifier tube I4 is adjustable to permit adjustment of the bias of tube 14 for a purpose which will become clear.

Illustrative values for the various elements of direct-coupled amplifier Ill and compensation circuit l2 have been shown in the drawing, and it is believed that the invention will be more readily comprehended if itbe assumed that the circuit is comprised of elements having the values shown. Itshould be understood, however, that the values shown are merely illustrative and that they are neither critical nor limiting. Y

For convenience of description, the adjustabl cathode biasing resistor [8 of tube 14- will be referred toas control X, the adjustable tap of potentiometer 23 will be referred to as control Y, and the adjustable tap of potentiometer 34 willbe referred-to as control Z, these reference designations being shown on the drawing. a

In describing the operation of the compensation means provided by the present invention, it will; be helpful to first consider what happ ns when variations occur inthe positive supply voltage, 3+. Notethat, as seen bythe positive volte sq r e rlr. there ar two unpaths o the. plate oftube H to ground. The first of these paths comprises the plate resistance of amplifier tube I4 and the cathode biasing resistor I8, and the second comprises the resistance-divider coupling circuit I6 and the negative voltage source C. When tube I4 is conducting, its D.-C. plate resistance may, in a typical case, be of the order of from say 50,000 to 250,000 ohms, depending upon tube bias. It will be seen, assuming the resistance-divider circuit I6 is comprised of elements having values of the order shown in the drawing, that the D.-C. impedance from the plate of tube I4 to ground through the conducting tube and cathode resistor 18 is much smaller than the impedance from plate to ground through resistance-divider circuit I6 and negative voltage source, In the circuit illustrated, the impedance of the resistance-divider path is from ten to fifty times as large as the D.C. impedance to ground by way of the space path of tube I4. Thus, for a given value of supply voltage and a given value for plate resist-or I1, the D.-C. voltage on the plate of tube I 4 is'determined primarily by the plate-to-cath-ode space resistance of tube I4, which in turn is determined by the operating bias. In other words, the D.-C. voltage on the plate of tube I4 is primarily a function of the adjustment of control X. This being true, the extent to which the plate voltage of tube I4 varies when variation-s occur in the positive supply voltage, B+, is likewise largely dependent upon the setting of control X. To illustrate, control X may be so adjusted that the plate voltage on tube I 4 is of the order of say one-half that of the supply voltage, B+. In that case, when the supply voltage deviates from its intended or static value, but one-half of the deviation will be felt at the plate of tube I4.

Since the voltage on grid 25 of tube I is afraction of the difference between the plate voltage on the plate of tube I4 and the negative supply voltage, C, the D.-C. voltage on grid 25, and the extent to which the potential of grid Varies with variations in B+, are primarily dependent upon the adjustment of control X. The potential of grid 25 is also, of course, dependent upon the adjustment of control Y but. the efiect of control Y is secondary.

Referring now to circuit I 2, it will be seen that when the positive supply voltage 13+ departs from its static value, thereby causing the grid potential of amplifier tube-I 5 to vary to an extent determined primarily by the adjustment of control' X, ashas just been described, a simultaneous variation occurs in the potential of grid 36 of compensation tube- 30. The current drawn by tube throu-ghcathode resistor 20 then changes insuch direction and to such an extent that the potential of cathode 2 I tends to follow the variation in the potential of grid 36. These variations in the potentials of grid-'36- and cathode ZI- are in the same direction as the variation occurring simultaneously in the potential of grid 25 as a resultof the variation in the positive supply voltage 3+. For a given position of control Z", control X could be so adjusted that the magnitude of the change occurring in the potential of grid 25, due to variation in the positive supply volt-age, B|-, is substantially equal to that occurring in the potential of cathode -2I. As a preferred matter, however, control X is so adjusted that when B+ varies a small change occurs in the grid-t-o-cathode potential of tube I5 to compensate for the fact that the plate potential of tube I5 also changes when B+ varies. In otherwords, withcontrol Z set at a given position, the plate-toground potential of tube I5 may, by suitable adjustment of control X, be maintained substantially constant despite variations in the positive sup-ply voltage B+. Thus, the output voltage level will be stabilized again-st variations in B+.

Let us now consider what happens when the negative supply voltage C deviates from its intended or static value. It will be seen that the static potential on grid 25 of amplifier tube I 5 is determined by the resistance-divider circuit I6 and by the voltage difference between the voltage on the plate of tube M and the negative supply voltage 0-. The extent to which the potential of grid 25 varies with variations in the negative supply voltage C is, however, primarily dependent upon the make-up of divider circuit I6 since, when C- varies, the voltage on the plate of. tube I4 remains relatively constant. In other words, the voltage at the plate of tube I4 may 'be considered as a point of substantially fixed potential, so far as. the negative voltage supply, C-, is concerned. Thus, when C varies, a fraction of the variation, determined by the resistancedivider circuit I6, appears at the grid of tube I5.

Referring again to circuit I2, when the negative supply voltage C- varies, the potential on grid 36 of compensation tube 30 and the potential at the cathode 2| of amplifier tube I5 vary in phase therewith, for a reason similar :to that previously indicated in connection with the discussion. of variations in the positive supply voltage B|. By suitable adjustment of control Z, the variations occurring in the potential of grid 25. of amplifier tube I5 due to fluctuations in the negative supply voltage 0- may be neutralized by substantially equal variations in the potential of cathode 2|.

In practice, the adjustments to controls X, Y and Z are preferably made in the following sequence: First, control Y is positioned to establish a desired outputlevel. Next, with control Y fixed, control Z i adjusted until the compensation is correct for variations in the negative supply voltage, C. And finally, with controls Y and Z fixed, control X is adjusted until the compensation is correct for variations in the positiv supply voltage, B+. If readjustment is necessary, the procedure may be repeated.

I have found that when the arrangement proposed by the present invention is employed, little, if any, variation occurs in the output voltage level when the negative and/or positive supply voltages deviate from their static values.

Thecircuit described is capable of continuous high speed corrections. If desired, the circuit may be adjusted to over-compensate in a desired direction, thereby to take care of variations in following stages due to power supply variations.

Having described my invention, I claim:

1. In a direct-coupled amplifier; a source of direct-current voltage subject to potential fluctuations; first and second amplifier :tubes each havingat least triode elements; a; compensator tube having at least triode elements connections from points of difierent potentials on said voltage source to the plate and cathode of each of said tubes for maintaining the plate of each tube at a more positive D.-C. potential than it cathode, said connections including an impedance common to the cathodes of said second amplifier tube and said compensatortube; means for coupling the output-0f said first amplifier tube to the input of said second amplifier tube, said means comprising an impedance connected from the plate of said first tube to a point of low potential on said voltage source and a connection from a tap on said impedance to the grid of said second tube, by reason of which connections the D.C. potential of said second-tube grid differs from that of said second-tube cathode by a selected biasing voltage, and by reason of which connections said second-tube grid is subject :to variation in its D.C. potential in response to said potential fluctuations of said source; a second impedance; connections from points of high and low potential on said source to the terminals of said second impedance for maintaining :the terminals of said second impedance at difierent D.C. potentials; and a connection from a tap on said last-named impedance to the grid of said compensator tube, by reason of which connection the D.C. potential on the grid of said compensator tube difiers from that on the cathode of said compensator tube by a selected biasing voltage and the potential of said compensator-tube grid varies in response to fluctuations in the potential of said source.

2. In a direct-coupled amplifier comprising:

first and second tubes each having at least cathsecond tubes; means biasing said first tube; a

cathode resistor biasing said second tube; means connecting the plate of said first tube conductive- 1y to the grid of said second tube; mean connecting the grid of said second tube conductively to said source of negative supply voltage; means for applying a signal to the grid-cathode circuit of said first tube; means for deriving an output signal in the plate circuit of said second tube; a third tube whose plate resistance i substantially lower than that of said second tube; means connecting said source of positive supply voltage to the plate of said third :tube; a resistance connected between said source of positive supply voltage and said source of negative supply voltage; an adjustable tap on said resistance; means connecting said tap conductively to the grid of said third tube; and a low impedance conductive connection between the cathode of said third tube and the cathode of said second tube, whereby when either of the supply voltages vary the potentials on the grid and cathode of said second tube vary in the same direction.

3. A direct-coupled amplifier stabilized against variations in power supply voltages, said amplifier comprising: first, second and third tubes each having at least cathode, grid and plate electrodes; a source of positive supply voltage; a source of negative supply voltage; means connecting said source of positive supply voltage to the plates of said first, second and third tubes; means biasing said first tube; means for applying a signal to the grid-cathode circuit of said first tube; first and second potentiometers each having a resistive element and an adjustable tap; means, including the resistive element of said first potentiometer, connecting the plate of said first tube conductively to said source of negative supply voltage; means connecting the tap of said first potentiometer conductively to the grid of said second tube; a common resistance connecting the cathodes of said second and third tubes to a point of fixed reference potential; means, including said resistive element of said second potentiometer, connecting said source of positive supply voltage to said source of negative supply voltage; means connecting the tap of said second potentiometer to the grid of said third tube; and means for deriving an output potential between the plate of said second tube and said point of fixed reference potential, said tap of said second potentiometer being so adjusted that for a given adjustment of said first potentiometer the output potential of said second tube remains substantially constant when said negative supply voltage deviates from" its static value, said first-tube biasing means being so adjusted that for a given adjustment of said first and second potentiometers th output potential of said second tube remains substantially constant when said positive supply voltage deviates from its static value.

4. A direct-coupled amplifier comprising: first and second tubes each having at least cathode, grid and plate electrodes; a source of positive supply voltage; a resistance connecting said source of positive supply voltage to the plate of said first tube; means for varying th operating bias of said first tube; means for applying a signal to the grid-cathode circuit of said first tube; a source of negative supply voltage; first and second potentiometers; means, including the resistance element of said first potentiometer, for connecting the plate of said first tube conductively to said source of negative supply voltage; means connecting the adjustable tap of said first potentiometer conductively to the grid of said second tube; means connecting said source of positive supply voltage to the plate of said second tube; a resistance connecting the cathode of 'said second tube to a point of fixed reference potential; a third tube having at least cathode, grid and plate electrodes, the plate resistance of said third tube being small relative to that of said second tube; means connecting said source of positive supply voltage to the plate of said third tube; means connecting the cathode of said third tube to said point of fixed reference potential by way of the cathode resistance of said second tube; means, including the resistance element of said second potentiometer, connecting said source of positive supply voltage conductively to said source of negative supply voltage; mean connecting the adjustable tap of said second potentiometer conductively to the grid of said third tube; and means for deriving an output signal between the plate of said second tube and said point of fixed reference potential, said tap of said second potentiometer and said first-tube biasing means being so adjusted that variations in said positive or negative supply voltages cause the cathode and grid potentials of said second tube to vary in the same sense and by amounts according substantially to a predetermined ratio.

5. Apparatus as claimed in claim 4 characterized by the fact that said tap of said second potentiometer and said first-tube biasing means are so adjusted that Variations in said positive or negative supply voltages cause the cathode and grid potentials of said second tube to vary in the same direction and by substantially equal amounts.

ROBERT P. NELSON.

REFERENCES CITED UNITED STATES PATENTS Name Date Goodale et al May 18, 1948 Number 

